Perpendicular magnetic recording more advantageous for high-density recording in principle than in-plane magnetic recording increases the recording density of a hard disk drive (HDD) by about 40% per year. It is probably not easy to achieve a high recording density even by using the perpendicular magnetic recording method because the problem of thermal decay becomes serious again.
“A high-frequency field assisted recording method” has been proposed as a recording method capable of solving this problem. In this high-frequency field assisted recording method, a high-frequency magnetic field much higher than a recording signal frequency and close to the resonance frequency of a magnetic recording medium is locally applied to it. Consequently, the medium resonates, and the coercive force (Hc) of the medium in the portion to which the high-frequency magnetic field is applied becomes half the original coercive force or less. By using this effect, it is possible, by superposing a high-frequency magnetic field on a recording magnetic field, to perform magnetic recording on a medium having a higher coercive force (Hc) and a higher magnetic anisotropic energy (Ku). If a high-frequency magnetic field is generated by using a coil, however, it becomes difficult to efficiently apply the high-frequency magnetic field to a medium.
As a high-frequency magnetic field generating means, therefore, a method using a spin torque oscillator has been proposed. In the disclosed technique, the spin torque oscillator includes a spin transfer layer, interlayer, magnetic material layer (oscillation layer), and electrode. When a direct current is supplied to the spin torque oscillator through the electrode, magnetization in the magnetic material layer ferromagnetically resonates due to spin torque generated by the spin transfer layer. As a consequence, the spin torque oscillator generates a high-frequency magnetic field. Since the size of the spin torque oscillator is about a few ten nm, the generated high-frequency magnetic field locally exists in a region of about a few ten nm in the vicinity of the spin torque oscillator. In addition, an in-plane component of the high-frequency magnetic field can efficiently resonate a perpendicularly magnetized medium, and largely decrease the coercive force of the medium. Consequently, high-density magnetic recording is performed in only a portion where the recording magnetic field of the main magnetic pole and the high-frequency magnetic field of the spin torque oscillator are superposed. This makes it possible to use a medium having a high coercive force (Hc) and high magnetic anisotropic energy (Ku). Accordingly, the problem of thermal decay in high-density recording can be avoided.
To implement a high-frequency field assisted recording head, it is important to design and manufacture a spin torque oscillator capable of stably oscillating with a low driving current, and generating an in-plane, high-frequency magnetic field that sufficiently resonates medium magnetization.
A maximum current density that can be supplied to a spin torque oscillator is, e.g., 2×108 A/cm2 when the element size is about 70 nm. If the current density is higher than that, the characteristics deteriorate due to, e.g., the heat generation and migration of the spin torque oscillator. Therefore, it is important to design a spin torque oscillator capable of oscillating at as low a current density as possible.
On the other hand, to sufficiently resonate medium magnetization, it is reportedly desirable to set the intensity of an in-plane, high-frequency magnetic field at 10% or more of the anisotropic magnetic field (Hk) of the medium. Examples of a means for increasing the intensity of the longitudinal high-frequency magnetic field are increasing the saturation magnetization of an oscillation layer, increasing the thickness of the oscillation layer, and increasing the rotational angle of magnetization in the oscillation layer. Unfortunately, any of these means increases a driving current.
As described above, decreasing the current density of the driving current is inconsistent with increasing the intensity of the in-plane, high-frequency magnetic field, so a spin torque oscillator meeting these conditions at the same time is desirable.